The present invention relates to a method and apparatus for continuously withdrawing from waste gases containing particulate matter, moisture and condensates, a clean gas sample for use in monitoring or analysis.
Waste gases which are produced from industrial processes such as chemical recovery boilers, waste incinerators etc. contain chemicals and combustion products which when released into the atmosphere pollute the environment. Control and optimizing of these processes in order to obtain a desirable balance of combustion products and waste gases is achieved by sampling of the waste gas and analysis of the gaseous components contained therein, preferably on a continuous basis. Known methods of withdrawing gaseous samples from such waste gases encounter problems in dealing with the particulate matter, moisture and condensate vapours which clog and corrode the sampling apparatus. In addition the condensates present may be carried over with the gas sample to foul monitoring or analysis apparatus or render the gas sample incomplete by absorption of part or all of the gas sample constituents to be monitored or analyzed. Ideally, a sampling apparatus should deliver an intact gas sample representative of the source atmosphere and having a free flowing rate sufficient so that when it is monitored it reflects the process parameters as the system is operating and at the time of monitoring or analysis. In addition, the sampling apparatus itself should be constructed of a material or combination of materials which will resist corrosion by the source atmosphere and gas sample such that maintenance of the gas sampling probe is significantly reduced or eliminated. Some known methods of continuously withdrawing a gas sample from a source of waste gas have employed a high velocity jet of steam for aspiration of sample gases through a probe and for driving the resultant steam-gas mixture into a chamber where water is added to the steam-gas mixture to condense the steam about the foreign matter in the gas sample. The resultant foreign matter whose effective mass is increased by the condensed steam is separated from the gas to be analyzed by a centrifugal separator. Using water in this way results in the formation of corrosive liquids which can attack the sampling apparatus. The water also affects the contents of gaseous components by introducing into the sample gas oxygen from the water and dissolving CO.sub.2, SO.sub.2, H.sub.2 S, NO.sub.x and other components of interest from the gas sample in the water.